There are many structural elements in an aircraft that require the use of additional pieces to improve the mechanical behavior of these structural elements. These additional pieces can be stiffeners in particular. These stiffeners are profiled parts attached to the structural elements of the aircraft, for example to transfer loads longitudinally or to stabilize those elements (to prevent them from bubbling or buckling due to shearing or compressive forces).
Stiffeners can be used in the aircraft fuselage, for example, as frames or stringers to stiffen the skin and certain specific areas, such as the door frames. They can also be used in the wing units of the aircraft, in the sense of wing spars (longerons) or ribbing (ribs).
Stiffeners can stiffen the structure locally, in the vertical or longitudinal direction, at places where the stresses are high.
Stiffeners can have cross sections with different shapes. The cross section of the stiffener depends on many parameters, such as the shape of the structural element to be stiffened or the main function it is supposed to perform. The known stiffeners generally have a Z, T, J or Ω cross section. FIGS. 1, 2 and 3 show examples of different cross sections that are known.
More specifically, FIG. 1 shows an example of a stiffener with a Z-shaped cross section. Such a Z-shaped stiffener 2 has 3 parts: a cap 21, a core 22 and a head 23. The cap 21 is in contact with the element to be stiffened 1 and thus takes its shape. The core 22, which has two “plane” surfaces, can potentially serve to hold other elements. The head 23 serves to stabilize the stiffener, that is, to prevent its section from tilting into its plane.
FIG. 2A shows an example of a T-shaped stiffener. This T-shaped stiffener 3 has two parts: a cap 31 and a core 32. The cap 31 is in contact with the element being stiffened and thus takes its shape. The core 32, which has two “plane” surfaces, can potentially serve to mount other elements. As a composite, such a stiffener could, for example, be obtained by RTM technology (Resin Transfer Molding) by co-injecting two L-shaped performs positioned back to back, as shown in FIG. 2B.
In these two examples of stiffeners, like a J-shaped stiffener, the stiffener has a core that forms a supporting surface. A supporting surface is a plane surface that is perpendicular or quasi-perpendicular to the element being stiffened that can hold an additional element so that that element is in constant contact with the supporting surface. Thus, such a supporting surface can ensure that another element, structural or not, is mounted. In other words, these types of stiffeners with J, T or Z-shaped cross sections each have a plane surface onto which an additional element can be mounted.
However, the shape of these stiffeners can be a drawback when the structure is subject to certain forces. Indeed, when the stiffener is compressed, it has a tendency to tilt, i.e., it has a tendency to buckle when the compression reaches a certain level. To prevent the stiffener from buckling, it is necessary to add additional pieces, such as stabilization clips placed over the stiffener, whose role is to prevent their load from tilting.
To solve these stability problems, there are stiffeners with an Ω-shaped cross section. An example of such an Ω-shaped cross section is shown in FIG. 3. In the example in FIG. 3, this stiffener 4 has an Ω-shaped cross section. It has a head 45, two cores 43 and 44 and two caps 41 and 42, symmetrical on either side of the median plane of the head 45. The shape of the stiffener, and particularly the fact that combined with the element to be stiffened the section obtained is closed, a section similar to a hollow beam, gives said stiffener stability. In other words, the stiffener is stable by itself, without adding any additional parts. It is autostabilized. Thus, even if such a stiffener is compressed, it is easy to see that the stiffener will not be tilted to either one side or the other.
However, such an Ω-shaped stiffener has no simple supporting surface for other elements. Indeed, besides its two caps, such a stiffener has no surface really adapted to take other elements beyond orienting its head and its cores (none is perpendicular to the surface of the element being stiffened) and beyond problems associated with installing any fastener in closed sections (controllability, specific attachments . . . ).
Thus, depending on the structural element being stiffened and the functions being performed, the choice is to use a stiffener with a supporting surface, like a stiffener with a Z, T or J-shaped cross section or an autostabilized stiffener, like the stiffener with the Ω-shaped cross section.
Now, the current trend in aeronautics is that there are always more elements to mount the equipment elements and system elements. Particularly on the fuselage of an aircraft, the frames or the floor structure are areas of the aircraft where there are many elements to be added, both structural elements and system elements, such as electric cables. So it is important to allow elements to be mounted on the stiffeners by simplifying the structural parts to the maximum by integrating functions.